Measuring the velocity of a signal in a liquid permits determination of many different characteristics or its properties. The velocity of the signal traveling in the liquid often is referred to as the “sound velocity”, although frequencies of the signal energy measured can be different from typical sound frequencies in the audio range. The term “sound velocity” is used hereinafter to include all signal frequencies. Using the sound velocity together with appropriate mathematical relationships allows for determination of various characteristics or properties of the liquid such as its density, purity, concentration, components, etc.
Several different types of apparatus exist for measuring the sound velocity of a liquid present in a specimen. The term “specimen” is used hereinafter to define any receptacle or conduit having opposing walls, at least one of which is deformable, between which the liquid whose sound velocity is being measured is contained. The liquid in this specimen can be in a more or less static state or flowing through it. Examples of such specimens would be tissue of a human, such as an earlobe, in which blood is located in a substantially static state. Another is deformable plastic tubing in which a liquid is either static or flowing. Such tubing is used for a multitude of medical and industrial applications. The different types of apparatus for measuring sound velocity generally are classified as being of the contact or non-invasive type. In the contact type, some part or parts of the measuring apparatus come into direct contact with the liquid. In the non-invasive type, the sound velocity is measured without any part of the measuring apparatus coming into contact with the liquid.
In many applications it is preferred that the sound velocity measurement be made non-invasively so that the specimen does not have to be invaded and contact by an external member will not compromise the sterility or characteristics of the liquid. For example, in medical and biotechnology applications the non-invasive technique permits the sound velocity to be measured and from it various parameters of the liquid calculated using algorithms solved by a computer. The non-invasive technique permits this to be accomplished without having to invade the specimen to withdraw the liquid for analysis. Where blood is the liquid, typical parameters to be calculated are glucose and hematocrit values. The non-invasive technique also is useful to measure the sound velocity of hazardous chemicals and ultra pure liquids, such as are used in semiconductor processing systems. In such applications if an invasive technique is used where a part of the measuring apparatus comes in contact with the fluid, this might lead to contamination of the liquid and compromise its use in further processes. Also, when dealing with hazardous and corrosive fluids possible damage to parts of the measuring apparatus is avoided since there is no contact with the liquid.
Several instruments are known for making the sound velocity measurement non-invasively. For example, in U.S. pre-grant patent publication 2006/0052963 two pairs of ultrasonic transducers, or sensors, are used on opposing walls of the specimen. One of the sensors of each pair is a transmitter of ultrasonic (electro-mechanical) signal energy and the other is a receiver. The transmitting and receiving sensors of each pair are mounted on opposite sides of the specimen (tubing is shown) in which the fluid is flowing. The transit time of a signal from the transmitting sensor to the receiving sensor of each pair along a respective path through the fluid and the two specimen walls is measured. The sound velocity of the signal in the liquid is calculated from the results of the two transit time measurements. While such apparatus is effective in determining the sound velocity, it requires four sensors. Also, in some of the disclosed embodiments a special mounting is required for the sensors of the two pairs so that the transmitter and receiver sensors are offset at an angle from the tubing wall and from each other along the tubing length. Here the ultrasonic signal is transmitted by one sensor of each pair upstream and downstream of the fluid flow to the other sensor of the pair on the tubing opposite side.
In U.S. Pat. No. 7,481,114 a specimen in the form of flexible tubing is mounted in a fixture having a device that produces a force to deform the tubing external and internal dimensions at one point in a direction transverse to the tubing length. The tubing, or specimen, cross-sectional dimensions are hereafter referred to as “transverse length” since they are in a direction that is perpendicular to the tubing longitudinal axis and the fluid flowing in it. With the tubing not being deformed, there is a first acoustic path length in which a signal is transmitted by a transducer through both walls of the tubing and the transmitted signal is reflected back to the transducer. The force producing device is then operated to deform the normally circular flexible tubing cross section by a first amount to form a second acoustic path length. In the second acoustic path length a signal transmitted by the transducer is reflected from an inner wall of the deformed tubing back to the transducer. The round-trip transit times of the signal in both the first and second acoustic paths is measured and from this the patent disclosure indicates that the sound velocity can be computed although no specific teaching for doing this is given.
Non-invasive measurement has particular application when measuring the sound velocity of blood. For example, it is often necessary that one or more parameters of the blood of a person or animal be measured or monitored on a basis that requires a rapid determination of the parameter. For example, according to the International Diabetes Federation, more than 280 million people worldwide are currently living with diabetes and that number is expected to rise up to 438 million by 2030. People who have this disease must measure and monitor the level of glucose in the blood serum in order to control their disease, such as by change in diet or injection of a medication such as insulin. Presently, blood glucose most often is measured by taking a blood sample from a finger prick and applying the sample to an enzymatically medicated colorimetric test strip. The patient determines the glucose level based upon the color of the test strip. Many patients need to perform this procedure, which is annoying if not painful, several times per day. Also, the color measurement provides only a rough determination of the glucose level.
As another example, the measurement of the hematocrit blood parameter often is required. To accomplish this in the emergency/routine hospital environment, one or more blood samples are drawn from the patient, placed in vials and sent to a blood laboratory for analysis. Drawing of the blood causes the patient pain and discomfort. Also, since the blood has to be sent to a laboratory it means that the results are not available immediately. It often may require one hour or more waiting time. Patients requiring a rapid value measurement of a blood parameter, such as the hematocrit/hemoglobin value, sometimes are victims of a disaster events such as an auto accident. Such patients are undergoing trauma or are bleeding heavily in an emergency room. Accordingly, it would be desirable for medical personnel to react as promptly as possible when they need measurement of the hematocrit value to better react to the condition of the patient and sometimes even to save the patient's life. Therefore, it would be desirable to be able to determine the hematocrit value on a prompt basis that does not require drawing blood from the patient and sending it to a laboratory.
In yet another example where non-invasive measurement of sound velocity is advantageous, private practice physicians often need to screen a patient for possible anemia or other diseases. In such situations, a prompt determination of the hematocrit value is desirable so that a patient can be treated promptly, without having to draw a sample of the blood, wait for the laboratory results, and often have the patient make a return trip to the physician's office to learn the results and undergoing necessary treatment.
As should be apparent, a non-invasive technique for measuring sound velocity does not require extraction of blood from a patient. This advantageously does not subject the patient to the pain or discomfort caused by drawing blood to be placed in vials or by the pinprick extraction method. Also, by using the sound velocity measured by the non-invasive technique and appropriate computer based algorithms various blood parameters can be determined on a real time basis. This solves the problem of having to spend time to develop a colorimetric response and avoids having to send the blood to a laboratory to determine the value of the desired blood parameter.
Various non-invasive instruments have been developed to measure the blood glucose value using different technology approaches such as optical, electrical Impedance, thermal and electromagnetic technology. For example, LEIN Applied Diagnostics, an United Kingdom based company, has developed a non-invasive optical device that scans the human eye to measure glucose level. To date, none of the none-invasive approaches has been totally successful in providing an instrument for achieving accurate glucose measurement in a manner that is patient friendly in requiring no invasion of the body and that is easy to use, gives results with a degree of accuracy that is satisfactory for use, and that can be built at a relatively low cost. Attempts also have been made to develop instruments for non-invasively measuring the blood hematocrit parameter but none has been entirely successful in the combined aspects of ease-of-use, accuracy and low cost of manufacture.
Accordingly, it would be desirable to provide an apparatus and method that can be used to quickly and accurately determine the sound velocity of a liquid located or flowing in a specimen having at least one deformable wall.